Sundance 1976

Our planet coalesced from the gravitational attraction of rocky objects orbiting the sun around 4.5 billion years ago. Heavy impacts in the first millions of years vaporized oceans and the planetary surface multiple times. Possibly life evolved several times and was extinguished from these impacts. Early on a very large object (one third mass?) smashed into the earth and produced the moon, which is truly a piece of the earth. Almost as soon as cooling was sufficient and conditions stable, prokaryotic cellular life evolved, perhaps 3.5 billion years ago. For billions of years this was the only life on the planet. At that time there was no land, oceans covered everything 2 km deep, the continents formed later. All surface water is thought to be derived from comets. Several extinctions probably occurred without leaving any evidence. When the ability to photosynthesize was evolved and carbon dioxide was absorbed to produce oxygen, levels of the highly reactive gas began to rise in the atmosphere. It must have taken hundreds of millions of years for the first bacterial plants to raise levels of oxygen, particularly since all the dissolved iron first had to be oxidized. But eventually levels did start to rise where there had been no oxygen before, creating the first mass extinction caused by life, since many cells could not tolerate the conditions. This is thought to have occurred around 2-2.5 billion years ago. Oxygen is important because it seems to be a necessary condition for "animal" life. It is not known how or why multicellular life evolved, or why complex cells (eukaryotic) did, but it took a long long time, measured in the billions of years. The earth went through a horrific phase called snowball earth about 700 million years ago in which it essentially froze over. At that time the sun was cooler than it is now, allowing positive feedback glaciation effects to take place-- possibly due to the effects of early plants drawing down too much CO2 in the atmosphere. It is surprising that it didn't remain in that condition. The only way it could have warmed itself back up was through volcanism which slowly but surely brought the levels of greenhouse gases back up again. Without doubt this period was another mass extinction, lost to fossil history. Some time after rewarming came the cambrian explosion which resulted in a huge increase in diversity of body plans. Nonetheless another extinction, the biggest so far, occurred some 251 million years ago-- this one thought to be due to excess greenhouse gases creating an anoxic ocean and oxygen-depleted atmosphere. We learn there is no consistency in geological time-- gases go up and down, temperatures go up and down. Reptiles and proto-mammals subsequently evolved and competed head to head but in the early earth which was quite a lot hotter than today, average 15-20 degrees C hotter, the dinosaurs prevailed and ruled for 200 million years, until a freak accident: a 12 km wide rock hit the earth. This rock was like mount everest smashing into the Yucatan, when it struck the ocean its top was still sticking out the atmosphere. Every organism larger than about 10 kg died, for some reason all the dinosaurs preferentially excepting birds. The mammals took over, probably mostly by accident. Placental birth, caring for the young, warm-bloodedness, are likely preconditions for high intelligence mandating a mammalian evolution. The earth slowly cooled in the post-dinosaur period due to continental drift effects and drawing down of CO2 by plants. The last ice age of 2 millions years allowing cooler temperatures in Africa probably pushed intelligence a little higher (overheating the brain was always a problem for upright hominids on the savannah) to allow the evolution of homo sapiens. Currently a 6th large extinction is underway, having started about 70,000 years ago when the species left Africa for other continents. It will irreversibly lead to the extinction of most mammals, most birds, most reptiles, almost all amphibians, and probably most plants and insects. Temperatures in the next thousands of years are expected to rise at least 10 degrees, possibly more, leading us back to the hothouse earth of the dinosaur era. The next ice age will be easily averted due to human intervention and the melting of all ice on the poles. This will be the second big extinction known to have been caused by a form of life, this time by only one species though. Eventually after millions of years the earth will recover, and life will go on. What will happen next? We don't know, we will not be witnesses to this. Other forms of life will appear and thrive. Then, some 700 million years from now, the earth will be probably too hot (the sun heats as it ages), and that will be the end of the history of life here. So we can see from this that we are already in the old age of the biosphere, 80-90 percent of the history of life is in the past. It is hard to escape the metaphor of humanity being a metastatic cancer that is sickening an elderly patient (the planet).

Yea Yea...